GN09MGU

I assume this is probably a regulator fault but thought I would post on here anyway in case there is a TF quirk. My battery ran low a few weeks ago due to near zero car use. Charged it. All well. And from this I know the alternator was putting out about 13.8, because I measured it at the time. Today, a few hours after retrieving the car from the MOT tester, I noticed the alternator light was on. Checked the voltage - 13.2. Engine off. Engine back on. Light out. But voltage still 13.2.